1. Field of the Invention
The present invention relates to a capacitance type humidity sensor in which a moisture-sensitive film is disposed between two electrodes to increase permittivity of a capacitance between the two electrodes, and to a manufacturing method of the same.
2. Related Art
A humidity sensor such as the one disclosed in JP-A-60-166854 has a substrate as a lower electrode, a moisture-sensitive film formed on the substrate, and a thin upper electrode having moisture permeability. However, as the upper electrode is exposed to sensing conditions, it is required to have high resistivity against such conditions.
To the contrary, as is disclosed in JU-A-5-23124 or JPA-7-20080, a proposed sensor includes two opposing electrodes formed on an identical plane of a substrate, and a moisture-sensitive film is formed on these two electrodes. However, a resistance detection type sensor is mainly disclosed in these references while a capacitance detection type sensor is only briefly and generally described. Further, although these electrodes are disposed under the moisture-sensitive film, the electrodes are nonetheless exposed to water because moisture can reach the electrodes through the moisture-sensitive film. Therefore, a problem arises because the electrodes must be water resistant. Consequently, precious metal is preferably used to form the electrodes to ensure reliability of the electrodes against moisture. However, the precious metal increases manufacturing cost and becomes a contaminant in the manufacturing process.
Incidentally, the sensor disclosed in JU-A-5-23124 has a dielectric material as a base plate, and therefore a discrete circuit board is required. The sensor plate is electrically connected to the circuit plate by wiring that increases stray capacitance. Therefore, the capacitance in a detection portion should be designed so as to be large to increase the signal to noise ratio. As a result, the size of the sensor body inevitably increases.
An object of the invention is to provide a humidity sensor that has high reliability. Alternatively, another object of the invention is to provide a humidity sensor that can be produced in a semiconductor process line, and a manufacturing method of the same.
According to an aspect of the present invention, a first insulation film is formed on a semiconductor substrate. First and second electrodes are disposed on the first insulation film and oppose each other with a gap interposed therebetween. A second insulation film is formed so as to cover the first and second electrodes. Moreover, a moisture-sensitive film is formed so as to cover the second insulation film.
In this structure, the second insulation film is interposed between the first and second electrodes and the moisture-sensitive film, so that the electrodes are water resistant.
According to a second aspect of the present invention, a circuit portion is integrated in the semiconductor substrate in which a humidity sensing portion is formed. The circuit portion processes signals transmitted from the sensing portion.
Preferably, the first and second electrodes are composed of the same material as that of wiring in the circuit portion. Thus, the electrodes and the wiring are simultaneously formed.
Also, the circuit portion preferably includes a switched capacitor circuit that can detect small capacitance amounts.
In addition, the second insulation film is preferably composed of a silicon nitride film. The silicon nitride film has a relatively high dielectric constant, so that capacitance between the first and second electrodes increases.
Preferably, the first insulation film is composed of a material that has a dielectric constant lower than that of the second insulation film. Thus, parasitic capacitance between the electrodes and the substrate can be decreased.
Incidentally, a molecular organic material that has a high hygroscopicity, such as polyimide or butyric acetylcellulose, can be employed as the moisture-sensitive film.
Preferably, the respective electrodes have a comb-shaped pattern having electrode portions, and the electrode portions of the first electrode are interleaved with those of the second electrode to increase a facing area between the electrode portions of the first and second electrode.
Incidentally, the electrodes may be composed of a material such as Al, Alxe2x80x94Si, Ti, Au, Cu, and poly-Si, or a compound composed of at least two of those materials.
Preferably, the moisture-sensitive film is disposed so that a surface thereof contacting the second insulation film is located lower than top surfaces of the first and second electrodes, to thereby dispose the moisture-sensitive film in the gap formed between the electrodes.
Incidentally, the sensing portion is formed after the circuit portion is formed.
According to a third aspect of the present invention, a moisture-sensing base is mounted on a supporting substrate so that a signal having a change in capacitance that is detected in the moisture-sensing base is outputted through a wiring electrode portion formed in the supporting substrate. In this case, a moisture-sensitive film faces the supporting substrate, a connecting electrode portion for outputting a signal from the moisture-sensing base to the wiring electrode portion is disposed between the moisture-sensing base and the supporting substrate. The wiring electrode portion is protected from ambient conditions by a coating material.
Other features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings.